Why Do We Sleep?

p CHAPTER 12 Why Do We Sleep? A Clock for All Seasons Theories of Sleep The Origins of Biological Rhythms Sleep as a Passive Process Biological Clocks Sleep as a Biological Adaptation Biological Rhythms Sleep as a Restorative Process Free-Running Rhythms Sleep and Memory Storage Zeitgebers Focus on Disorders: Seasonal Affective Disorder The Neural Basis of Sleep The Reticular Activating System and Sleep The Neural Basis of the The Neural Basis of the EEG Changes Associated Biological Clock with Waking Suprachiasmatic Rhythms in a Dish The Neural Basis of REM Sleep Immortal Time Pacemaking Sleep Disorders Disorders of NREM Sleep Sleep Stages and Dreams Focus on Disorders: Sleep Apnea Measuring Sleep Disorders of REM Sleep A Typical Night’s Sleep Sleep and Consciousness NREM Sleep Focus on Disorders: Restless Legs Syndrome REM Sleep and Dreaming What Do We Dream About? Bob Thomas/Tony Stone Micrograph: Dr. Dennis Kunkel/Phototake 444 I p he polar bear, or sea bear (Ursus maritimus), has an sure to which they and we respond is similar. Our behav- amazing life style (Figure 12-1). As winter begins in iors are adaptations that maximize our ability to obtain T the Arctic and the days become shorter, the bears food and minimize the loss of energy stores that we obtain congregate to prepare for their migration north onto the from food. In other words, we are active during the day be- pack ice. Some bears travel thousands of kilometers. In the cause that is when we can obtain food, and we are inactive continuous darkness of the Arctic winter, the bears hunt at night to conserve body resources. Bears hunt all winter seals, walrus, and whales. While the bears are on the ice, to build up fat supplies, and they enter torpor so that they they take time to sleep; but their sleep cannot be called ei- can extend the period during which they can live on those ther nighttime or daytime sleep, because their world is fat stores. Hibernation is a strategy that rodents use to ex- continuously dark. At the same time as the bears are tend fat supplies for as long as possible. It is similar to shal- preparing to go out onto the ice, many other Arctic animals low torpor except that body temperature declines are so are escaping winter. Arctic terns fly 15,000 kilometers to extreme that the animals expend almost no energy. The mi- Antarctica, where it is summer. Lemmings, mice, and gratory behavior of birds is also a strategy used to maxi- ground squirrels cannot travel long distances; these rodents mize food acquisition and minimize energy loss, except spend the winter in burrows in a sleeplike state called hi- the objective is achieved by moving to a habitat where bernation. When summer comes again, the birds return food is abundant. and the rodents emerge from their burrows. The sea bears There is one other way in which we are similar to sea return from the ice, dig beds in the earth, and spend the bears, rodents, and birds. Our behaviors are not simply re- summer in sleep. sponses to the immediate changes that are taking place in The behavior of the sea bears is remarkable to us in our respective environments. We anticipate and prepare two ways. First, we are diurnal animals (from the Latin for the environmental changes that will result in food abun- diaes, meaning “by day”): we are active in the daylight, dance or food shortages. The sea bears are clearly prepared and we sleep when it is dark. Our recent evolutionary his- to go out on the ice well in advance of its formation be- tory places early humans in Africa at latitudes where day cause they walk along the Arctic shores for weeks before and night are almost equal in length. Because we are the ice forms; they also leave the ice before it melts. The adapted for daylight vision and have difficulty seeing any- birds migrate before food resources are depleted and win- thing at night, we prefer to avoid darkness. Therefore, it ter sets in. Rodents gorge themselves, build nests, and store seems strange to us that an animal would seek out and food in their burrows before winter arrives. We retire to flourish in darkness. Second, as diurnal animals, we obtain our food in daylight and we sleep for about 8 hours each night. Our sleep is characterized by a decline in body temperature and a loss of awareness of our surroundings. The sea bears sleep in the winter, as mentioned, but they will spend the entire summer in a condition of shallow torpor—a condition resembling sleep except that the decline in body temperature is greater than that during sleep. Their torpor appears to be voluntary because, if they have access to food throughout the year, they do not enter torpor. It is hard to imagine that we could voluntarily spend David Myers/Tony Stone David Myers/Tony all summer in a sleeplike condition. Figure 12-1 Despite the very great differences between our behav- In winter polar bears migrate to the Arctic ice, where they hunt in ior and the behavior of sea bears, the environmental pres- darkness and periodically sleep. I 445 p 446 I CHAPTER 12 sleeping sites in preparation for sleep, and we frequently ing questions related to biological rhythms and sleep: How get up before it is fully light to prepare for our daily activi- is our behavior modified to cope with the day–night cycle? ties. Because we, along with other animals, appear to have Why have we chosen sleep as a strategy for waiting out the warning of impending winter or impending changes in the night? What neural mechanisms regulate sleep and wak- day–night cycle, there must be signals to which we all re- ing, and what disorders develop when those mechanisms spond. In this chapter, we will seek answers to the follow- are disrupted? A CLOCK FOR ALL SEASONS To anticipate daily and seasonal changes, we have biological clocks that respond to cues in our environment. In this section, we will consider the cues that guide our behavior. Because environmental cues themselves are not always consistent, we will examine the role of biological clocks in interpreting environmental cues in an intelligent way. We will also discover how our internal biological clocks adjust our behavior to maintain our schedules. Figure 12-2 Daily light–dark changes are produced by the daily rotation of the earth on its axis The Origins of Biological Rhythms in which each part of the earth faces the The daily and the seasonal changes displayed by animals are called biological rhythms. sun for part of the rotation cycle These rhythms are in turn related to the rhythmic cycle of days and seasons produced (daytime) and faces away from the sun by the earth’s rotation on its axis and the earth’s progression around the sun (Figure for the other part (nighttime). Seasonal 12-2). The earth rotates on its axis once every 24 hours, producing a 24-hour cycle of changes in amount of daylight are day and night. The axis of the earth is inclined slightly, and so, as the earth orbits the related to the annual movement of the sun, the North Pole is tilted slightly toward the sun for part of the year and is tilted earth around the sun. Because the axis slightly away from the sun for the rest of the year. When inclined toward the sun, the of the earth is tilted, one pole points toward the sun for part of the year Southern Hemisphere experiences summer and gets more direct sunshine for more (summertime) and away for the other hours each day and the climate is warmer. At the same time, the Northern Hemisphere, part of the year (wintertime). inclined away from the sun, receives less direct sunlight making the days shorter and Seasons change as the earth revolves around the sun. Each revolution Northern winter takes 1 year. Northern summer Sun The tilt of the earth on its Day and night result from rotation of axis determines seasons. the earth on its axis every 24 hours. p WHY DO WE SLEEP? I 447 the climate colder. Tropical regions, being near the equator, undergo little climatic change as the earth progresses around the sun. Because of the seasonal differences in polar and equatorial regions, animals living near the poles are relatively more affected by seasonal changes and animals living near the equator are relatively more affected by day-and-night changes. In addition, the seasonal and daily changes may have combined effects on organisms, inasmuch as the onset and duration of daily changes depend on the EXPERIMENT season and latitude. Animals living in the polar regions also have to Question: Is plant movement exogenous or endogenous? cope with greater fluctuations in daily temperature, light, and food Procedure availability than do animals living near the equator. (A) The movements of the A pen attached to a We humans are equatorial animals in that our behavior is gov- plant’s leaves are recorded leaf is moved when erned more by daily cycles than by seasonal cycles. Our behavior is in constant dim light. the leaf moves,… dominated by a rhythm of daylight activity and nocturnal sleep. Not only does human waking and sleep behavior cycle daily, so also Revolving drum do pulse rate, blood pressure, body temperature, rate of cell divi- sion, blood cell count, alertness, urine composition, metabolic rate, sexual drive, and responsiveness to medications. But humans are Pen not unique in this respect.

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